Our group looks at two aspects of H2AX. The first involves elucidating the roles thatthis protein plays in DSB recognition, DNA rejoining and chromatin remodeling. The second utilizes the finding that H2AX phosphorylation is several orders of magnitude more sensitive than previous methods of DSB detection, making it a uniquely useful means of uncovering novel roles for DSBs in cellular metabolism. Our current program encompasses both basic and applied research projects and focuses on three projects. The first is a translational project investigating the parameters necessary to make H2AX a useful biodosimeter in humans and other animals. Two recent developments have given considerable impetus to this project. The first development is the finding by us and others that &amp;#947;-H2AX is formed in tissue culture cells as a response not only to agents that directly cause DNA DSBs but also to those that indirectly cause DSBs by interfering with cellular metabolism. As many of the agents used for cancer treatment fall into this latter category, this finding greatly increases possible roles for &amp;#947;-H2AX as a biomarker for drug responses. The second important development is the formation of a multi-disiplinary NCI team to expedite the clinical evaluation of new therapeutic and imaging agents, so-called phase zero trials. &amp;#947;-H2AX formation is being examined as a possible biodosimeter in these studies. We are also examining &amp;#947;-H2AX formation in mouse models and are involved in several clinical protocols either approved or being approved in the phase zero and phase one trials. This work is important because it will permit clinicians to obtain immediate feedback from the cells of an individual patient, feedback which can then be used to tailor the treatment to that patient, thereby improving their survival. The ultimate goal of this initiative is to develop &amp;#947;-H2AX detection into a useful tool for human health. The second project involves the use of &amp;#947;-H2AX to probe for interactions between cells in animals. In vitro studies by us and others have shown that cells that have been exposed to ionizing radiation affect the viability and the presence of DNA damage products in unexposed cells that contact the exposed cells or media from the exposed cells, a controversial phenomenon known as the bystander effect. We have shown that a robust bystander effect is present in artificial human skin tissue. Working from the hypothesis that the bystander effect is an example of a larger phenomenon of communication among damaged and healthy cells, we have demonstrated that media from tumor cells induces responses in bystander cells indistinguishable from those induced by media from irradiated cells, and are currently demonstrating the ability of other stresses to induce bystander responses. In addition, we are finding that this communication can be demonstrated in the intact animal. The presence of a human tumors implanted into nude mice leads to higher levels of DNA damage in certain other cells in the mouse. We are determining when and how this communication takes place in the animal and how this might be useful for human health. This work is important because it may increase the ways information can be obtained from individual patients. It is possible that by monitoring certain easily obtained tissues of a patient, information can be obtained about events in other less accessible tissues, perhaps leading to early detection of disease and cancer. The third area focuses on the structure and dynamism of the &amp;#947;-H2AX focus. We are developing tools that will permit us to study focal substructure and how it changes with time. We have shown for example, that in yeast, Mre11 does not bind to the whole &amp;#947;-H2AX domain, but is concentrated next to the break site. Studies such as these will complement other findings concerning the interactions of various proteins with &amp;#947;-H2AX, leading to a greater understanding of DNA DSB repair and the maintenance of genome stability. This work is important as iot will provide the basis for understanding the important parameters involved in utilizing &amp;#947;-H2AX foci as a biodosimeter.